Cellular Aging: DNA Polymorphisms

All DNA polymorphisms that affect the phenotype of the organism are expressed in some fashion at the cellular level, even if only in the secretion of an abnormal gene product that acts at a site distant from the secretory cell. Alleles of genes involved in basic cellular functions could affect the phenotype of multiple cell types, or in some cases all cell types. For example, polymorphisms in genes involved in DNA synthesis or cell division could alter the function of cells that are actively proliferating or are potentially capable of proliferation. On the other hand, a variant gene involved in an essential function such as aerobic respiration could potentially alter the phenotype of every cell in an organism.

The replication and repair of nuclear (genomic) DNA involves a variety of functions that are essential for cell survival. These metabolic processes are essential for the accurate transmission of genetic information from one generation of a cell or organism to the next, and for the maintenance of normal gene function. Diminished fidelity of DNA replication and/or repair will result in an increased mutation rate, which can lead to decrements of cell function, cell death and/or increased risk of transformation to a malignant (cancerous) cell type. There have been a number of experimental observations that suggest (but do not prove) that the accumulation of genomic mutations in somatic cells is a causal mechanism of aging both at the cellular and organismal levels. This hypothesis, generally attributed to Szilard, implies that the efficiency and fidelity of DNA replication and repair affect the rate of aging and maximum life span. Some of the observations that are consistent with this hypothesis are:

1. The efficiency and extent of the repair of DNA damage induced by ultraviolet light (UV) is directly related to the maximum life span of the species. This result correlates with the observation that lower levels of DNA damage and mutations are present in experimental animals that are on a diet that restricts caloric intake. Dietary restriction has been shown to extend maximal life span in multiple species and has been extensively exploited as an experimental model in aging research.
2. Normal human cells in tissue culture can divide only a limited number of times. This is often referred to as replicative senescence and is associated with changes in cell structure and gene activity. It now appears that one mechanism that determines the replicative potential of some cell types in cultures and in tissues is the extent of loss of specialized hexanucleotide repeat sequences of DNA (telomeres) at the ends of the chromosomes. Approximately fifty to one hundred bp are lost from this region with each cell division. This occurs because telomerase, the enzyme that synthesizes these repeat sequences, is functionally inactive in most human somatic cells. It has been postulated that after a sufficient number of replications these structures become so short that the cells perceive them as damaged DNA and irreversibly cease cell division.
3. The Werner syndrome, a rare genetic disease that is associated with decreased longevity and many features (but not all) of premature aging, is caused by a mutation in a helicase gene. This class of genes catalyzes the unwinding of the double helix of DNA, which is necessary for a number of essential functions, such as DNA replication and repair and messenger RNA transcription. Cultured cells derived from individuals with Werner syndrome display numerous abnormalities in their chromosomes and complete fewer cell divisions before the onset of replicative senescence than cultures derived from normal (non-Werner) individuals.
4. Some investigators have recently reported that the frequency of gene mutations in cell populations in the body increases with age. These observations are consistent with a diminution of the fidelity of DNA replication and/or repair with advancing age. Alternatively, this age-associated increase of mutation frequency could merely reflect a steady accumulation through time. At the present time there is no definitive evidence for the existence of decrements in the efficiency and/or fidelity of DNA replication or repair with advancing age.

There are a large number of proteins involved in the replication and repair of DNA. At this time there are 125 genes known to be directly involved in DNA repair. The products of these genes perform many specific functions in the repair process, including: recognition of damaged sites (DNA binding proteins); excision of the damaged region (exonucleases and endonucleases); replication of a new strand following excision of the damaged area (polymerases); and ligation of the newly synthesized segment of the strand (ligases).

Following completion of the first draft of the human genome, the identification of polymorphic alleles in these loci has proceeded very rapidly. For example, as of July 2001, 252 SNPs had been identified in genes that are associated with DNA repair. Moreover, more genes that code for proteins that are involved in DNA repair are being discovered. Even allowing for some inaccuracies at this time in the current Utah database, the frequency of this class of DNA polymorphisms is such that the existence of alleles with differing functional activities is almost a certainty.

The identification of specific alleles or groups of alleles (haplotypes) that effect the aging phenotype and/or longevity will involve multiple experimental approaches, as described above. If specific allelic associations are shown to be associated with some aspect of aging or longevity, the next step will be to establish a causal relationship between the alleles and the aging process. These studies will include a biochemical characterization of each allele to identify functional alterations, such as increased enzymatic activity. Transgenic technology—the insertion or deletion of genes into the germ line of experimental animals—will certainly play a pivotal role in establishing a cause-and-effect relationship between specific alleles or haplotypes and the aging phenotype.